The invention relates to a data compression apparatus for data compressing at least a first and a second digital information signal, each of the at least two digital information signals comprising subsequent samples, the apparatus comprising:
means for receiving the first and second digital information signal, PA1 signal combination means for combining the first and second digital information signal so as to obtain a composite information signal, PA1 data compression means for data compressing the composite information signal so as to obtain a data compressed composite information signal, PA1 output means for supplying the data compressed composite information signal, to a data expansion apparatus for data expanding a data compressed composite information signal obtained from at least a first and a second digital information signal, the apparatus comprising PA1 input means for receiving the data compressed composite information signal, PA1 data expansion means for data expanding the data compressed composite information signal so as to obtain a data expanded composite information signal, PA1 retrieval means for retrieving a replica of the first and second digital information signal from the data expanded composite information signal, PA1 output means for supplying the replicas of at least the first and second digital information signals, to a transmitter comprising the data compression apparatus, to a receiver comprising the data expansion apparatus, to a method for data compression, to a method for data expansion and to a record carrier.
Data compressing a stereo audio signal is well known in the art. Reference is made in this respect to EP-A 402,973, document D1 in the list of related documents. The document describes a subband coder, in which each of the left and right hand signal components of the stereo audio signal is A/D converted with a specific sampling frequency, such as 44.1 kHz, and the resulting samples in de form of eg. 24 bits wide words of the audio signal, are supplied to a subband splitter filter. The subband splitter filter splits the audio signal component into a plurality of relatively narrow band subband signals. Using a psycho acoustic model, a masked threshold is derived and blocks of samples of the subband signals are subsequently quantised with a specific number of bits per sample for each block of the subband signals, in response to said masked threshold, resulting in a significant data compression of the audio signal to be transmitted. The data compression carried out is based on `throwing away` those components in the audio signal that are inaudible and is thus a lossy compression method.
In an intensity stereo mode, a further data reduction can be obtained by combining, in the same subband, samples of the subband signals of the left and right hand signal components of the stereo audio signal, by adding them so as to obtain samples of a composite subband signal and carrying out a data compression on the composite subband signal.
The data compression described in document D1 is a rather intelligent data compression method and requires a substantial number of gates or instructions, when realized in hard or software respectively, so that it is expensive. Moreover, the subsequent expansion apparatus also requires a substantial number of gates or instructions, when realized in hardware or software respectively.